Wavelength-division multiplexed (WDM) optical networks often require dynamic filters at nodes that can have each channel either pass the node without distortion or be diverted into the node and provide a replacement. Examples of such filters integrated onto a single chip have been described. (See K. Okamoto et al., "16-channel optical add/drop multiplexer consisting of arrayed-waveguide gratings and double-gate switches," Electron. Lett., vol. 32, pp. 1471-1472, 1996; C. G. M. Vreeburg, et al., "First InP-based reconfigurable integrated add-drop multiplexer," IEEE Photon. Technol. Lett., vol. 9, pp. 188-190, 1997; L. Eldada et al., "Integrated multichannel OADM's using polymer Bragg grating MZI's," IEEE Photon. Technol. Lett., vol. 10, pp. 1416-1418, 1998; C. K. Madsen et al., "A multi-port add/drop router using UV-induced gratings in planar waveguides," Optical Fiber Communications Conference, Paper ThH3, pp. 104-106, 1999; and C. R. Doerr et al., "Arrayed waveguide lens wavelength add-drop in silica," IEEE Photon. Technol. Lett., vol. 11, pp. 557-559, 1999.)
Such known single-chip add/drop filters suffer from several substantial limitations including waveguide crossings, low channel count and difficulty in matching and controlling component characteristics. Large numbers of waveguide crossings require more chip real estate, leading to larger devices with lower channel counts and complex topologies.